The nitric acid–ammonia system is studied by high level ab initio calculations. The equilibrium structure, vibrational frequencies, and binding energy of the system in the gas phase are calculated at the second-order Mo/ller–Plesset perturbation level with the extended basis set 6-311++G(d,p). The potential energy surface along the proton transfer pathway is investigated by calculations at the same level of theory, and the effect of water as a solvent on the structure and stability of the system is investigated using self-consistent reaction field theory. It is found that the equilibrium structure contains a strong hydrogen bond with nitric acid acting as the hydrogen bond donor and ammonia as the acceptor. The binding energy is calculated to be D0=12.25 kcal/mol (De=14.26 kcal/mol), which is about three times greater than the binding energy for the water dimer. The OH stretching frequency of nitric acid in the hydrogen-bonded complex is found to be red shifted by over 800 cm−1, with an enhancement of over an order of magnitude in the infrared intensity from the isolated nitric acid molecule. The structure of ammonium nitrate corresponding to the product of a proton transfer reaction is found to be highly unstable on the potential energy surface. The most energetically favorable gas phase reaction of nitric acid and ammonia in the absence of a solvent results in proton exchange, not proton transfer from acid to base. The structure and stability of the system change drastically in the water solvent medium. In the water solvent, the hydrogen-bonded structure is no longer stable and the system exists as an ammonium nitrate ion pair resulting from the completed transfer of a proton from nitric acid to ammonia. On the basis of these results, we conclude that it is unlikely to form gaseous ammonium nitrate from nitric acid and ammonia, and that the formation of particulate ammonium nitrate most likely involves a heterogeneous mechanism.

The gas-phase proton-transfer reaction of ammonia−hydrogen chloride and the effect of the first three water molecules are investigated by high-level ab initio calculations on the molecular clusters NH3−HCl−(H2O) n , n = 0, 1, 2, 3. The equilibrium structures, binding energies, and harmonic frequencies of the clusters as well as the potential energy surfaces along the proton-transfer pathway of ammonia−hydrogen chloride are calculated at the second-order Møller−Plesset perturbation (MP2) level with the extended basis set 6-311++G(d,p). Either without water or with one water molecule, the ammonia−hydrogen chloride system exists as a usual hydrogen-bonded complex. With two or three water molecules, the system becomes an ion pair resulting from the complete transfer of a proton from hydrogen chloride to ammonia. The potential energy surfaces along the proton-transfer pathway are examined to understand the effect of the water molecules. The harmonic frequencies and infrared intensities of the clusters provide additional evidence in support of the transition from the hydrogen bond to the ion pair structure as the water molecules are stepwise introduced. On the basis of these results, we conclude that ammonium chloride might be formed by the gas-phase reaction of hydrogen chloride with ammonia in the presence of adequate water vapor.

The gas-phase proton-transfer reaction between ammonia and the hydrogen halides HF, HCl, and HBr and the influence of one, two, and three water molecules are investigated with high-level ab initio calculations on molecular clusters of NH 3 -HX-(H 2 O) n , n ) 0, 1, 2, 3 with X ) F, Cl, Br. Equilibrium geometries, dissociation energies, HX harmonic frequencies, and potential energy surfaces along the proton-transfer pathway of NH 3 -HX are calculated at the second-order Møller-Plesset perturbation (MP2) level with the extended basis set 6-311++G(d,p). It is found that although the NH 3 -HX dimer exists as a hydrogen-bonded structure for X ) F, Cl, and Br, it can be converted to an ion pair in the presence of water molecules. One water molecule is sufficient to promote a proton transfer from HBr to NH 3 , resulting in the ion pair NH 4 + ‚‚‚Br -, whereas at least two water molecules are required to induce a proton transfer from HCl to NH 3 . Three water molecules appear to promote a partial proton transfer from HF to NH 3 . The potential energy surfaces along the protontransfer pathway demonstrate the energy difference present between two forms of each cluster in which the NH 3 -HX system exists either as a hydrogen-bonded unit or as an ion pair NH 4 + ‚‚‚X -. The successive addition of water molecules to the system gradually increases the stability of the ion pair relative to the hydrogenbonded form. The potential energy curves, along with the geometry data and HX vibrational frequencies of the clusters, show how the progressive addition of water molecules affects a particular ammonia-hydrogen halide cluster and also indicate what trends exist between different ammonia-hydrogen halide clusters associated with the same number of water molecules.

No abstract

An exercise on the application of chemometrics for data interpretation, appropriate for a quantitative analysis course, is presented. Standard chemometric techniques such as principal component analysis and cluster analysis were applied to a multivariate data set of 14 river sediments samples containing ten heavy metals. The exercise was carried out by the students of a quantitative analysis class. The exercise allowed the students to develop the basic skills regarding how to extract meaningful information from experimental data.